How to resize an image in C# to a certain hard-disk size?

Question

How to resize an image an image in C# to a certain hard-disk size, like 2MiB? Is there a better way than trial and error (even if it's approximate, of course).

Any particular keywords to search for when trying to find the solution on the web?

Answer 1

You can calculate an approximate information level for the image by taking the original image size divided by the number of pixels:

info = fileSize / (width * height);

I have an image that is 369636 bytes and 1200x800 pixels, so it uses ~0.385 bytes per pixel.

I have a smaller version that is 101111 bytes and 600x400 pixels, so it uses ~0.4213 bytes per pixel.

When you shrink an image you will see that it generally will contain slightly more information per pixel, in this case about 9% more. Depending on your type of images and how much you shrink them, you should be able to calculate an average for how much the information/pixel ration increases (c), so that you can calculate an approximate file size:

newFileSize = (fileSize / (width * height)) * (newWidth * newHeight) * c

From this you can extract a formula for how large you have to make an image to reach a specific file size:

newWidth * newHeight = (newFileSize / fileSize) * (width * height) / c

This will get you pretty close to the desired file size. If you want to get closer you can resize the image to the calculated size, compress it and calculate a new bytes per pixel value from the file size that you got.